In the so-called HSPA (High Speed Packet Access) method of wideband code division multiple access (WCDMA) mobile phone communication, a number of different coding rates can be used. In addition, multi-level modulation can be used. Link adaptation is a method in which modulation and coding schemes by which a base station communicates with a user are adaptively selected based on the existing signal conditions being experienced by the user of the cellular communication system. By using link adaptation, an appropriate coding rate and modulation level (number of bits transmitted per signaling period) can be selected for each user. For example, in ordinary quaternary phase shift (QPSK) modulation, one can transmit two bits per phase transition, using four constellation points, with the amplitude remaining constant. With 8PSK, there are eight constellation points, making possible transmitting three bits per transition. With quadrature amplitude modulation (QAM), the amplitude can also be changed. For example, using 16 QAM, there are four possible phase values and four possible amplitudes leading to sixteen constellation points.
The prior art teaches changing coding and modulation based on a pilot signal level indicating a S/N ratio (i.e. the coding and modulation being changed based on a S/N measurement of the pilot signal). In response to an indication of a poor S/N ratio, the prior art teaches as one response changing to a lower modulation level or to stronger coding (more robust) and conversely in response to an indication of a good S/N ratio.
One problem with the prior art solution is that the pilot signal measurement includes several sources of error, and the algorithm used in the prior art to decide on what link adaptation to make (if any) does not work particularly well because of the many possible errors. For example, the algorithm may select the wrong modulation and coding set (MCS) if there is error in the pilot S/N measurement (or other similar measurements, such as an Ec/I0 or Eb/I0 measurement, which are both measurements made on the common pilot channel, CPICH, Ec/I0 standing for energy per chip to interference ratio, and Eb/N0 standing for energy per bit to interference ratio), or in case of an error in the prior art measurement of the HS-DSCH channel quality instead of the common pilot channel (CPICH) quality.
What is needed is a robust and fast link adaptation methodology for use by a mobile phone communicating via a wireless communication system allowing coding rate and modulation level to be changed. In some communication systems, such as HSDPA, a fixed power level is used for HS-DSCH, but for other the power level varies. Ideally, what is needed in general (for systems other than HSDPA) is a methodology that is not limited to use when the mobile phone is transmitting at either its maximum or minimum power level, but one that can be used when the mobile phone is transmitting also at any intermediate power level.